General collection efficiency in liquid iso-octane and tetramethylsilane used as sensitive media in a thimble ionization chamber

Johansson, Bengt; Bahar-Gogani, Jalil; Wickman, G.

doi:10.1088/0031-9155/44/9/304

Cited by 2 publications

(3 citation statements)

References 19 publications

(19 reference statements)

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“…The correction for general recombination ranges from 6% to 4% for the dose rates involved on the first and the last calibration occasion respectively. The uncertainty in the calculated correction factors at the dose rates involved on the first calibration occasion is estimated to be less than 0.2% (one standard deviation) (Johansson et al 1999) for any of the chambers and decreasing with time due to the decreasing amount of general recombination.…”

Section: Correction For General Recombinationmentioning

confidence: 94%

“…The standard uncertainty in the general collection efficiencies is estimated to be less than 0.2% (Johansson et al 1999).…”

Section: Uncertainty Analysismentioning

confidence: 96%

“…In absorbed dose determinations in electron and photon fields, since the polarizing voltage is the same as at the calibration of the LIC, only the general recombination has to be corrected for. The general recombination rate of TMS, isooctane and the mixtures used as the sensitive media in parallel-plate and thimble type chambers have previously been measured and compared with theoretical expressions in continuous and pulsed radiation (Johansson et al 1995, 1999. The compilation of the published data indicates that the general recombination at dose rates up to about 100 mGy min −1 (the range that the present chambers are intended for) should be less than 3% for LICs with 1 mm liquid thickness operating at a field strength of 0.3 MV m −1 .…”

Section: General Recombination (Dose Rate Dependencementioning

confidence: 99%

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Long-term stability of liquid ionization chambers with regard to their qualification as local reference dosimeters for low dose-rate absorbed dose measurements in water

Bahar-Gogani

Grindborg²,

Johansson

et al. 2001

Phys. Med. Biol.

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The long-term sensitivity and calibration stability of liquid ionization chambers (LICs) has been studied at a local and a secondary standards dosimetry laboratory over a period of 3 years. The chambers were transported several times by mail between the two laboratories for measurements. The LICs used in this work are designed for absorbed dose measurements in the dose rate region of 0.1-100 mGy min(-1) and have a liquid layer thickness of 1 mm and a sensitive volume of 16.2 mm3. The liquids used as sensitive media in the chambers are mixtures of isooctane (C8H18) and tetramethylsilane (Si(CH3)4) in different proportions (about 2 to 1). Operating at a polarizing voltage of 300 V the leakage current of the chambers was stable and never exceeded 3% of the observable current at a dose rate of about 1 mGy min(-1). The volume sensitivity of the chambers was measured to be of the order of 10(-9) C Gy(-1) mm3. No systematic changes in the absorbed dose to water calibration was observed for any of the chambers during the test period (sigma < 0.2%). Variations in chamber dose response with small changes in the polarizing voltage as well as sensitivity changes with accumulated absorbed dose were also investigated. Measurements showed that the LIC response varies by 0.15% per 1% change in applied voltage around 300 V. No significant change could be observed in the LIC sensitivity after a single absorbed dose of 15 kGy. The results indicate that the LIC can be made to serve as a calibration transfer instrument and a reference detector for absorbed dose to water determinations providing good precision and long-term reproducibility.

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